This invention relates in general to a process for removing adherent material, such as paint, scale or other coverings from hard surfaces and, more particularly, to a non-chemical surface cleaning process employing blast cleaning using an abrasive which is propelled to the surface in a pressurized fluid stream. For various types of structures, it is often necessary or desirable to remove any layer of coating which has been previously applied to or formed on surface areas. Numerous techniques exit for removing paint, sealants, lacquers, rust, scale and other adherent materials from virtually any type of surface. Surface cleaning or stripping methods range from mechanical abrasion to the use of strong chemicals and involve varying degrees of time, effort and expense. For any given type of coating, the character and function of the substrate material from which a coating is to be removed usually dictates the stripping method, at least in industrial settings.
In view of the environmental and health hazards involved in the use of solvents for cleaning surfaces, in particular, large exterior surfaces, it has become common practice to use an abrasive blasting technique wherein abrasive particles are propelled by a high pressure fluid against the solid surface in order to dislodge previously applied coatings, scale, dirt, grease or other contaminants. Various abrasive blasting techniques have been utilized to remove coatings, grease and the like from solid surfaces. Thus, blasting techniques comprising dry blasting which involves directing the abrasive particles to a surface by means of pressurized air typically ranging from 30 to 150 psi, wet blasting in which the abrasive blast media is directed to the surface by a highly pressurized stream of water typically 3,000 psi and above, multi-step processes comprising dry or wet blasting and a mechanical technique such as sanding, chipping, etc. and a single step process in which both air and water are utilized either in combination at high pressures to propel the abrasive blast media to the surface as disclosed in U.S. Pat. No. 4,817,342, or in combination with relatively low pressure water used as a dust control agent or to control substrate damage have been used. Water for dust control has been mixed with the air either internally in the blast nozzle or at the targeted surface to be cleaned and such latter process, although primarily a dry blasting technique, is considered wet blasting inasmuch as media recovery and clean up is substantially different from that utilized in a purely dry blasting operation. Hard, durable surfaces, such as heavy steel plating can be cleaned or stripped by a hard abrasive such as sand. Softer metals such as aluminum or more delicate surfaces such as polymer composite layers may require the use of a softer abrasive material during blasting such as plastic pellets or sodium bicarbonate.
Sand blasting of steel plate or other hard surface to remove adherent coatings and the like, while successful in removing the coatings, has several disadvantages. For one, the sand abrasive is very friable such that upon contact with the surface, a vast amount of silica dust is formed. Recently there has been a concern that the minute air-borne free-silica particles which are formed during blasting present a substantial health hazard, in particular, if ingested into the lungs. Secondly, very large amounts of sand are required for cleaning large structures such as bridges, stacks, etc. such that after blasting, this sand remains and must be removed from the blast cleaning area adding substantially to the time and expense of the blasting process.
Alternative abrasives for blast cleaning hard surfaces are known. For example, U.S. Pat. No. 3,775,180 is directed to a method for descaling steel in which the steel is descaled by spraying a mixture of a solid such as aluminum oxide or silicon carbide with water and a gas such as air under specified conditions onto the steel. In removing a coating or a scale on the surface of a metal, however, it is important that the anchor pattern (surface roughness) of the metal surface be uniform and not too extensive such that the surface and even the metal structure is damaged. A blast media composed only of hard aluminum oxide and silicon carbide can be detrimental to the metal structure. For certain surfaces such as metals softer than steel, a softer abrasive can be used with the blast stripping method. An example of such is disclosed in U.S. Pat. No. 4,878,320 to remove coatings from aluminum, fiber glass or carbon fiber laminate. As disclosed in the patent, an abrasive particle is used which has a Mohs hardness of about 3. Sodium bicarbonate is the preferred material. Likewise, the present assignee markets a sodium bicarbonate blast media under the tradename Armex.RTM. for removing paint, scale, dirt, grease and the like from aluminum, stainless steel, and non-metallic surfaces. A large advantage of sodium bicarbonate as a blast media is that the material is water soluble and non-toxic so that a blasting area can be cleaned relatively easily by washing the abrasive blast media away with water without a harmful environmental effect. Although very effective for removing coatings from aluminum or other softer materials such as polymer composites and the like, sodium bicarbonate abrasive is not hard enough to provide a sufficient anchor pattern on hard surfaces such as steel so as to prepare the surface for the addition of a new coating layer.
Other patents which disclose cleaning metal surfaces with an abradant other than sand include U.S. Pat. No. 2,624,988 which utilizes a mixture of Tripoli paste and a liquid vehicle to which mixture can be added sponge rubber fragments which carry the abradant to the metal surface and which provide a rubbing action to polish and buff the metal surface. The addition of a small amount of alumina to the Tripoli paste is disclosed under certain conditions.
U.S. Pat. No. 2,710,286 discloses a method of removing fluorescent and other materials from viewing screens of cathode ray tubes in which sodium and potassium carbonate are used as the abrasive material. U.S. Pat. No. 4,588,444 discloses removing calcium from polymeric contact lenses by using as an abradant sodium chloride, sodium bicarbonate or a mixture of same. U.S. Pat. No. 4,731,125 discloses a method for removing adherent material from composite surfaces made of a reinforced matrix material using a granular media composed of particles which have a Mohs hardness of lower than 3.5. Preferably the abradant is polymeric particles having the desired Mohs hardness.